The present invention relates to radiation detectors, particularly to planar semiconductor radiation detectors, and more particularly to electron gas grid semiconductor radiation detectors which utilize at least delta-doped layers adjacent to the anode of the detector to form an internal free electron gas grid.
Various types of radiation detectors have been developed for detecting gamma-rays and x-rays, among which are the planar semiconductor radiation detectors. Semiconductor radiation detectors generally operate by absorbing a quantum of gamma-ray or x-ray radiation and by converting the radiation energy into a number of electron-hole pairs that is proportional to the absorbed energy. After the conversion, the motion of the electrons and holes induce electrical signals on the detector electrodes. The electrical signals are also proportional to the energy of the absorbed radiation. Hence, by using a semiconductor radiation detector, one can detect gamma-ray and x-ray radiation and measure its energy spectrum.
The conventional planar semiconductor radiation detector, such as illustrated in FIG. 1, does not function well due to the poor electrical transport properties of the holes. Many of the common radiation detectors are made from CdZnTe or GaAs, with a cathode and anode made for example of gold, and, for these semiconductors, the electrical signal due to the holes is much typically smaller than the electrical signal due to the electrons. These effects are due to the slower motion of the holes and greater probability of trapping of the holes within these materials. Because the total electrical signal is a sum of the signal due to the electrons and the holes, the signal due to the holes complicates the signal analysis and results in poor energy resolution and efficiency in these detectors.
The planar semiconductor radiation detector also suffers from a position dependence on the signal. For example, a signal due to electrons originating from radiation absorbed near the cathode will be larger than a signal originating from near the anode. Thus, the conventional planar semiconductor radiation detectors suffer from both poor electrical transport properties of the holes and from a position dependence of the signal.
Recent efforts have been directed to improve the energy resolution of the planar semiconductor radiation detectors and also to lessen the dependence of the signal on the position of the radiation absorption, and thus allow one to isolate the electrical signal from the electrons. These improved approaches are referred to as xe2x80x9celectron-only devicesxe2x80x9d and have shown to give superior energy resolution for x-ray and gamma-ray radiation over the conventional planar semiconductor radiation detectors. The xe2x80x9celectron-only devicesxe2x80x9d are exemplified by P. N. Luke, xe2x80x9cSingle-polarity charge sensing in ionization detectors using coplanar electrodes,xe2x80x9d Appl. Phys. Lett. 65 (22), Nov. 28, 1994; E. Y. Lee, et al., xe2x80x9cDevice Simulation of an Unipolar Gamma-Ray Detector,xe2x80x9d Mat. Res. Soc. Symp. Proc., 487, p. 537 (1998), U.S. Pat. No. 5,677,539, issued Oct. 14, 1997 to B. Apotovsky, et al., U.S. application Ser. No. 09/075,419 filed May 8, 1998, entitled, xe2x80x9cMethod and Apparatus for Electron-Only Radiation Detectors from Semiconductor Materialsxe2x80x9d by Lund, et al., and U.S. application Ser. No. 09/075,351 filed May 8, 1998, entitled, xe2x80x9cHigh Resolution Ionization Detector and Array of Such Detectorsxe2x80x9d by McGregor, et al. These xe2x80x9celectron-only devicesxe2x80x9d place a third metallic electrode, called a grid, on the surface of the detector near the anode to electrostatically shield the anode from the signal originating between the grid and the cathode. FIG. 2 illustrates an embodiment of the prior xe2x80x9celectron-only devices,xe2x80x9d which is a unipolar gamma-ray detector of above-referenced Lee, et al. In these devices, all the signals from the anode originate from a motion of the electrons and holes moving between the anode and the grid. Since the electrons move toward the anode while the holes move away from the anode toward the cathode, due to their polarities, the signal on the anode will be dominated by the motion of the electrons. Furthermore, the signal will have much less position dependence, since electron trapping between the grid and anode is unlikely.
In an xe2x80x9celectron-only devicexe2x80x9d one can characterize the space between the grid and the cathode as a detection volume and the region between the grid and the anode as the measurement volume, as shown in FIG. 2. Ideally, all radiation absorbed in the detection volume would give rise to electrical signals due only to the motion of the electrons in the measurement volume. However, there are several imperfections associated with the prior art of the xe2x80x9celectron-onlyxe2x80x9d detector, which are:
1. For the grid to shield the anode, the grid can not be placed too close to the anode. This decreases the detection volume of the detector and therefore the radiation detection efficiency of the detector.
2. Many of the electrons created between the grid and the cathode are collected by the grid and produce no signal on the anode. Hence, these detectors have dead regions where no signals can be detected, leading to a loss of detector efficiency.
3. The internal electric field of the detector is highly non-uniform, due to the placement of the external grid. The electric field is uniform only very close to the cathode and the anode. The non-uniformity of the electric field causes variation in the charge collection time of the electrons. Since electron trapping does occur in the detector, this non-uniformity of the electric field results in variation of the signal strength with the position of the x-ray and gamma-ray absorption event, and hence in loss of the energy resolution. Attempts at correcting for the electron trapping by trying to deduce the position of the original radiation absorption are difficult due to the nonuniform internal field. This is commonly attempted by monitoring of the cathode signal and using it to correct the anode signal with electronic circuits external to the detector.
The present invention provides a detector which removes the three above-described imperfections in the xe2x80x9celectron-onlyxe2x80x9d detectors, resulting in superior energy resolution and radiation detection efficiency. In addition, the detector of the present invention has all the virtues of the xe2x80x9celectron-onlyxe2x80x9d detector, exploiting the excellent transport properties of electrons over holes and having signals that are independent of the position of the interaction. The present invention is an electron gas grid semiconductor radiation detector which employs doping of the semiconductor and variation of the semiconductor detector material to form a two-dimensional electron gas and to allow transistor action within the radiation detector. Superior energy resolution and radiation detection sensitivity over the prior art xe2x80x9celectron-only devicesxe2x80x9d are provided by the present invention. The detector of the present invention utilizes delta-doped layers adjacent to the anode which form an internal free electron gas grid, with or without a quantum well located between two adjacent doped layers, whereby an external electrode can be attached to either a doped layer or the quantum well.
It is an object of the present invention to provide an improved planar semiconductor radiation detector.
A further object of the invention is to provide an electron gas grid semiconductor radiation detector.
A further object of the invention is to provide an improved planar semiconductor radiation detector which employs doping of the semiconductor material adjacent the anode of the detector and employs variation of the semiconductor material to form a two-dimensional electron gas and to allow transistor action within the radiation detector.
Another object of the invention is to provide an improved semiconductor radiation detector which overcomes the problems associated with the prior art xe2x80x9celectron-onlyxe2x80x9d radiation detectors but which exploits the transport properties of electrons over holes and the position dependent signal of the prior art xe2x80x9celectron-onlyxe2x80x9d radiation detectors.
Another object of the invention is to provide an electron gas grid semiconductor radiation detector (EGGSRAD) which produces superior energy resolution and radiation detection sensitivity over all previously known semiconductor radiation detectors.
Another object of the invention is to provide an EGGSRAD which has in its structure delta-doped layers which function as atomically thin charged sheets, which are deposited during the growth of the semiconductor, and which produce an internal free electron gas grid to which can be attached an external grid.
Another object of the invention is to provide an EGGSRAD which in addition to the delta-doped layers includes a quantum well layer located between adjacent doped layers and of a material different from the semiconductor material, which produces an internal electron gas grid and results in better anode shielding capacity.
Another object of the invention is to provide an EGGSRAD which includes a graded bandgap by varying the composition of the semiconductor near the delta-doped layers and can be utilized with or without a quantum well layer.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings. The improved planar semiconductor radiation detector of the present invention broadly constitutes an electron gas grid semiconductor radiation detector (EGGSRAD) which includes four (4) structural embodiments which are described hereinafter as: 1) an elementary electron gas grid semiconductor radiation detector (EEGGSRAD), 2) a modulation doped EGGSRAD (MODEGGSRAD), 3) a graded bandgap EGGSRAD (GRABEGGSRAD), and 4) a graded bandgap MODEGGSRAD (GRADMODEGGSRAD). Each of the four embodiments employ delta-doped layers which produce an internal free electron gas grid for shielding the anode. In the MODEGGSRAD, a quantum well layer is located between two of the doped layers to form better shielding capacity of the electron gas grid, is composed of an undoped material different than the material of the semiconductor, and the two-dimensional electron gas forms in the undoped quantum well and not in the delta-doped layer. The GRABEGGSRAD and the GRABMODEGGSRAD, each include a varied composition of the semiconductor material near the delta-doped layers to produce spatial variation of the conduction band energy. All of the four EGGSRAD structures or embodiments have in common: 1) an electron gas grid, 2) the same basic features (the delta-doped layers), and 3) the same mode of operation. The EEGGSRAD embodiment is the simplest. The MODEGGSRAD uses modulation doping to improve the grid shielding capacity, and the GRABEGGSRAD and the GRABMODEGGSRAD use graded bandgaps to further optimize the detector performance by enhancing control of the electron transmission through the grid. Since these four structures or embodiments operate in the same way, they are collectively termed EGGSRAD.